1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor in which inner electrodes are formed by defining gap layers in a ceramic body and injecting molten lead or lead alloy in the same.
2. Description of the Prior Art
An example of the monolithic ceramic capacitor in the aforementioned structure is disclosed in U.S. Pat. No. 4,071,880 issued on Jan. 31, 1978 to Rutt. FIG. 5 shows the structure as disclosed in this prior art.
The monolithic ceramic capacitor as shown in FIG. 5 is formed by pluralities of alternately piled ceramic layers 1 and inner electrodes 2, and double-layer outer electrodes 3a, 4a, 3b and 4b are connected with prescribed ones of the inner electrodes 2.
A description will now be presented of a method of manufacturing such a monolithic ceramic capacitor with reference to FIGS. 6 to 9.
Ceramic green sheets 11A and 11B of, e.g., 50 to 100 .mu.m in thickness shown in FIGS. 6 and 7 are prepared by a doctor blade coater, to print carbon paste 21 prepared by mixing carbon powder and ceramic powder on the surfaces of the ceramic green sheets 11A and 11B. Two types of thus obtained ceramic green sheets 11A and 11B as shown in FIGS. 6 and 7 are alternately piled in plural, to be integrated with each other by application of pressure FIG. 8 shows a green (i.e., before firing) ceramic chip 12 thus obtained. Then the ceramic chip 12 is fired at a temperature over 1000.degree. C. to sinter the ceramic sheets while quenching carbon powder contained in the printed carbon paste 21, thereby to define porous gap layers of the ceramic powder in portions to form the inner electrodes 2 shown in FIG. 5. Thereafter the porous outer electrodes 3a and 3b are applied as shown in FIG. 9. The outer electrodes 3a and 3b are obtained by baking, e.g., paste mainly composed of nickel and mixed with glass frit. The ceramic chip 12 thus obtained is introduced in a pressure vessel to be dipped in molten lead of about 330.degree. to 360.degree. C. in a decompressed state and then pressurized to about 10 atm, thereby to inject, under pressure, the molten lead into the gap layers of the ceramic chip 12 by penetration through the porous outer electrodes 3a and 3b. Then the ceramic chip 12 is lifted up from the molten lead to be cooled and returned to normal pressure, whereby the inner electrodes 2 of lead are formed. Thereafter the outer electrodes 4a and 4b, which are excellent in solderbility, are applied to the surfaces of the outer electrodes 3a and 3b as needed.
In manufacturing of the aforementioned monolithic ceramic capacitor, the porous outer electrodes 3a and 3b are formed in advance of injection of the inner electrodes 2 so as to prevent lead from flowing out from the gap layers in the ceramic chip 12 which is lifted up from the molten lead after injection. In other words, the porous outer electrodes 3a and 3b serve as penetrable barriers. The porous outer electrodes 3a and 3b are mainly formed of nickel, which is non-reactive with lead, and their adhesion to the ceramic sheets 11, i.e., the ceramic chip 12, depends on the amount of glass frit contained in the paste, and adhesion strength thereof is improved by increasing the amount of glass frit. In this case, however, void ratios of the outer electrodes 3a and 3b are lowered, which obstructs penetration of the injected lead, while glass components block the gap layers to prevent introduction of the lead. Thus, electrostatic capacitance cannot be obtained of the designed value even if pressurization is made with sufficient pressure.